1. Field of the Invention
This invention relates to a process for recovery of chromium from aqueous solutions, such as spent plating solutions and rinse waters used in the electroplating industry. The process is a total recovery and recycle method for chromium from concentrated aqueous solutions which generates no chromium waste as in other known processes for chromium recovery, such as precipitation and ion exchange.
2. Description of Prior Art
Chromic acid solutions are used for chromium plating, for the electrolytic stripping of copper, for the anodizing of aluminum, and for the passivation of cadmium, magnesium, and zinc. The chromic acid baths are contaminated by the buildup of trivalent chromium, formed as a result of the reduction of chromic acids, and other heavy metals such as aluminum, zinc, copper, and iron, which dissolve from the pieces being treated. These contaminants have several detrimental effects on the plating solution including increased voltages and plating times, increased pitting and noduling, and decreased current efficiency. In addition, as a result of increased levels of contamination, the quality of the plating deteriorates and a portion of the bath must be discarded, even though it still contains large amounts of chromic acid. Spent plating solutions from these processes present a major waste treatment problem.
Known methods for the recovery and use of hexavalent chromium include ion exchange, reverse osmosis, electrodialysis, ion flotation, and chemical means. Known methods for the removal and destruction of hexavalent chromium in industrial effluents include reduction-precipitation, selective pyrolysis, electro-coagulation, incineration, organic complexation, foam and ion flotation, and chemical methods.
U.S. Pat. No. 5,112,583 teaches a process for recovering chromic anhydride from exhausted aqueous chromium plating solutions by alkalizing the exhausted chromium plating liquid to a pH comprised between three (3) and seven (7) at a temperature of 70.degree. C. with a commercial sodium hydroxide solution, keeping the mass in reaction at this temperature for an hour, followed by filtration to separate the precipitate formed. The filtered solution contains CrO.sub.3 which is then reduced with sulfur dioxide to obtain a solution containing basic chromium sulfate and sodium sulfate, suitable for use as a tanning bath.
U.S. Pat. No. 4,704,259 teaches a process for the removal of chromium in the form of chromate from electrolytically produced aqueous chlorate solutions by the formation and precipitation of Cr.sup.III as Cr(OH).sub.3 by reduction of hexavalent chromium with a dithionite. In particular, hydroxyl ions are added to an aqueous chlorate solution in a sufficient amount to provide alkaline pH conditions and to permit trivalent chromium to form Cr(OH).sub.3, followed by an addition of dithionite to cause precipitation of Cr(OH).sub.3, after which the precipitated Cr(OH).sub.3 is separated from the aqueous chlorate solution.
U.S. Pat. No. 4,446,026 teaches a process and solvent for extraction of chromates in an aqueous solution in which the aqueous phase is brought into contact with an organic phase comprising 2-10% by volume of a solvent of tertiary or quaternary amines and a solvent of esters of phosphoric acid in a diluent based on petroleum hydrocarbons.
U.S. Pat. No. 4,349,514 teaches a process for recovery of reusable chromic acid from a waste chromic acid solution containing impurities in which the solution containing the chromic acid is introduced into the cathode compartment in an electro-dialysis process and free chromic acid or chromate ions in the solution are transferred to the anode compartment. H.sub.2 SO.sub.4 is added to the heavy metallic ion contained in the outlet solution, in which chromic acid ion concentration is reduced, in order to convert chemical species of the contained metallic ion and increase the H.sup.+ ion concentration. Chromate ions in the resultant solution are extracted into an organic phase by contact with an organic solvent. Residual amounts of chromic acid ion remaining in the aqueous raffinate are extracted into an organic phase by a second organic solvent.
U.S. Pat. No. 5,120,523 teaches a method for dissolving a metal, including chromium, by contact of the metal with a quaternary ammonium compound and a halogenated hydrocarbon or with a quaternary ammonium compound, a halogenated hydrocarbon and polar solvent.
U.S. Pat. No. 5,271,910 teaches a process for treating metal chloride wastes produced by chlorination of titanium ore in which the metal chloride wastes are leached in a hydrochloric acid-containing solution; dissolved metals are separated from the remaining solids in the solution to obtain a liquid and a residue; the metals are then selectively precipitated as their hydroxides by adding a neutralizing agent to the liquid; and the metal hydroxides thus obtained are separated from the liquid to produce a residue which is subsequently dewatered.
U.S. Pat. No. 5,260,039 teaches a process for two-phase extraction of metal ions from phases containing solid metal oxides in which the solid metal oxides are brought into contact with a hydroxamic acid, dissolved in a hydrophobic organic solvent, the phases containing the solid metal oxides and the organic phase being intensively mixed. After phase-splitting, the organic phase is separated and the metal ions are re-extracted from the organic phase in a known manner.
U.S. Pat. No. 4,242,127 teaches a method for processing a hydroxide sludge residue to recover nonferrous metal values therefrom which includes the steps of concentrating the residue and acidifying and heating the concentrated residue to produce a solution containing the nonferrous metals followed by separation of the nonferrous metals, including chromium, by high temperature pressure hydrolysis to obtain the metals as hydrated oxides or basic sulfates.
Finally, U.S. Pat. No. 4,092,439 teaches a process for preparation of chromium dioxide by oxidizing hydrated trivalent chromium oxide. See also West, T. S., "Liquid-Liquid Extraction Procedures in Inorganic Analysis", Metallurigia, Page 47, July 1956, which presents a general overview of liquid-liquid extraction procedures in inorganic analysis including methods for extraction of chromium.
Of all of the known methods, ion-exchange is considered to be the most efficient means of hexavalent chromium recovery and reuse from rinse solutions. However, this method is not feasible at higher concentrations of chromic acid. Reduction-precipitation is generally considered the easiest means of destruction of hexavalent chromium in aqueous solutions. However, this method is very expensive; there is no recovery of chromium; and large volumes of hydroxide wastes are generated. To the extent that chemical methods to oxidize Cr(OH).sub.3 sludge to reusable chromium (VI) have been investigated, they have been found to be unfeasible. To our knowledge, at the present time, there is no viable recycle and recovery technique for the commercial reprocessing of spent chromium plating solutions, particularly those which are high in chromium concentration.